Fluid turbulence study apparatus and method

ABSTRACT

A rotating arm carrying vortex generators generates outwardly spiralling vortices in a fluid-medium. Measurements are made in the fluid disturbances caused by the vortices.

United States Patent [19] Schwind 3,709,036 Jan. 9, 1973 ReferencesCited UNITED STATES PATENTS 1,039,889 10/1912Brianne..;...........,...................

[54] FLUID TURBULENCE STUDY APPARATUS AND METHOD 73/147 Fales 11/1969Bart....................................272/31A ma RC n [73] Assignee:Nielsen Engineering and Research Primary Examiner-James J. GillCorporation Attorney-Limbach, Limbach & Sutton [57} ABSTRACT A rotatingarm carrying vortex generators generates outwardly spiralling vorticesin a fluid-medium. Mea- [22] Filed: Aug. 31, 1970 211 App]. No.: 68,310

surements are made in the fluid disturbances caused by the vortices.

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I e m llnllll n/ A 7 4L6 nu n |||||L w INVENTOR.

PATENTED MP1 9 I975 sum 1 0F 2 FIG 1 RICHARD G. SCHWIND M M 5 ATTORNEYSFLUID TURBULENCE STUDY APPARATUS AND METHOD BACKGROUND OF THE INVENTIONThe vortices that trail off the wing tips of an aircraft 0 cause itswake to persist and be dangerous to other airplanes that pass through itseveral minutes later. Present testing methods for obtaining data of thevortex persistency consist of probing the wake with another aircraft ortaking measurement in a scaled-down windtunnel experiment. In the formercase, it is very difficult to find the vortex center and obtainmeaningful measurements. For the latter type of experiment, existingwind tunnels do not have nearly as long or as large a test section as isneeded to measure their persistency and decay. The vortices are inclinedat an angle to the main flow and soon approach too closely to one of thewind-tunnel walls, so either this effect or the test section lengthlimits how much of the trailing vortices can be studied.

The recent introduction of markedly larger aircraft has reemphasized theproblem of the persistency of aircraft trailing vortices. Extensivestudy is in process at various laboratories. Because of the turbulentnature of the vortices, theoretical methods are based on empiricalturbulent transport information obtained from wellcontrolled modeltesting. However, wind tunnel measurements of a single trailing vortexare available only for relatively short distances downstream of theirpoint of generation. The farthest downstream a pair of trailing vorticeshas been measured is 13 chords in work published in 1926 by Page andSimmons.

In view of these problems it is desirable to have a relativelyinexpensive facility to produce vortex pairs and allow them to bemeasured for long distances downstream of their point of generation.

Trailing vortices result from rolling up of a wake behind a liftingsurface. For wings with monotonically decreasing lift from thecenterline out to the wing tip, there are only two vortices. Theycontain the vorticity trailing off the lifting surface, and for zeroangular acceleration have equal values of circulation, but opposite insense. In a few wing span lengths, the vortex sheets are well rolled upand the tangential velocity variation is then essentially independent ofthe method of generation, except for a virtual origin effect. The vortexcores then grow, with vorticity diffusing outward, but the circulationremains constant for contours entirely enclosing either vortex. In stillair, according to one hypothesis, the vorticity diffuses to the plane ofsymmetry, and starts cancelling with vorticity of opposite sense fromthe opposite vortex. This process continues until, theoretically, allvorticity is completely cancelled. In turbulent air, gross convection ofsegments of the trailing vortices can occur. It is anticipated thatturbulence may decrease the time that the velocities from the vorticesare dangerously high. Thus, the critical case of the trailing vortexpersistencey problem occurs when the free-stream air turbulence is verylow. This present invention deals principally with this case, as itsexperimental measurement and understanding is essential before any moregeneral turbulent atmosphere case can be treated.

A distinction between the present invention and a helicopter or fanblade should be considered. The blades of the latter devices produce aslipstream motion in which the trailing vortices are only incidentaland, in fact, these vortices will dissipate quickly in very turbulentconvective flow in which they are entrained. In an embodiment disclosedherein the rotating arm has no lift (as does a helicopter blade) andthus produces no slipstream motion; the arms purpose is to hold a wingsection or vortex generating pairs which produce pairs of essentiallysymmetric trailing vortices which move together through the quiescentfluid medium and persist for relatively long times.

SUMMARY OF THE INVENTION strength and they will move radially outward byself-induced motion. The vortex strengths and generator spacings can beadjusted so that when the wing or vortex generators return to anyspecific point of the circle the newly generated sections of thevortices will have negligible interference from any part of the vortexgenerated previously. Thus a very long trailing pair is generated whichspirals continuously outward from the wing or vortex generators forseveral or many turns before turbulent break-up or viscous dissipationoccurs.

The vortex generatons may consists of any device for creating a swirlingflow, such as a series of vanes, a tangential jet inside a shroud, or asmall electric motor driving a fan. They would typically consist of 1%to 3- inch diameter tubes. Different screens could be placed over theentrance to the vortex generators to change the drag of the vortices.

Measurements could be made in any of several ways. A stationaryplatformcould be placed in various positions to obtain unsteady measurements ofthe vortex pair passing by it. A second means would be to move thisplatform radially outward with the vortex pair induced velocity. Thiswould be equivalent to observing the wake of a passing aircraft at afixed position. Next, the arm could be extended outward from the vortexgenerators to obtain steady-state measurements one or more cycles afterits generation. A still further means would be to have a rotatingplatform which could obtain steady-state measurements at any position inthe vortex trail without creating the fluid dynamic disturbance upstreamof the measurement point as for the previous case.

The apparatus can be used to generate vortices in any fluid. It is notlimited to use in air or other gas, but is equally applicable 'toproducing trailing vortices in liquids. Screens could be used around theapparatus to dampen the nature fluid turbulence. A solid circular wallcould be placed concentrically around the spinning source of thevortices to simulate the ground so as to deflect the vortices just asthey are during aircraft take-off and landing. At any desired positionin the trailing vortex swirl the vortices could be removed from the areaby one of several means, such as a curved counterrotating deflectionplate or suction port. The

orientation of the wing axis or the axis between the pair of vortexgenerators can be arbitrary. If it is vertical,

the vortex pair trails off, and spirals outward due to the self-inducedvelocity as depicted in FIG. 1. In essence, the effect which moves thevortices toward the walls in a wing tunnel and limits their usefulnessfor vortex investigation is harnessed in the apparatus of the inventionto generate long spiral vortices. Since the vortex pair should spiraloutward to avoid interference with the arm, the wing should be mountedwith an angle of attack so thatits lift is directed radially inward. Thevortex pair could also be directed at nearly any angle to the arm exceptradially inward. It is expected, though, that the radially outward casedescribed will be most desirable because of its symmetry of vortex flow.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view oftrailing vortices.

FIG. 2 is a schematic diagram of the trailing vortex apparatus accordingto one embodiment of the invention.

FIG. 3 is a cross-sectional view of the rotating arm of FIG. 2.

FIG. 4 is a partially cut-away perspective view of one type of vortexgenerator.

FIG. 5 is a partially cut-away perspective view of a further type ofvortex generator.

FIG. 6 is a perspective view of a wing-type vortex generator.

FIG. 7 is a partial plan view of the apparatus of FIG. 2 showing amodified type of turbulence measurement.

FIG. 8 is a partial plan view of the apparatus of FIG. 3 showing afurther modified type of turbulence measurement.

FIG. 9 is a partial view of FIG. 2 showing a modified vortex trap.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referringnow to FIG. 1 of thedrawings wherein a schematic perspective view of vortices 16 trailingdownward and away from two pairs of vortex generators are shown. A shaftsupports a tapered arm 4 that has an array 6 of four vortex generators8, 10, 12 and 14, mounted at the end thereof. Arm 4 rotatescounterclockwise. Vortex generator pairs that are vertical to the .planeof arm rotation will generate vortices that spiral outward in the sameplane; vortex generator pairs mounted horizontal will produce vorticesthat spiral downward, perpendicular to the plane of arm rotation. Byselecting the angle between the vortex generator pair (such as 8 and 12)and theplane of arm 4 rotation, the direction in which the vorticesspiral away can be varied. In the schematic representation of FIG. 1,the generator pairs 8-12, 10-14 are at an angle of roughly from thevertical, thus there is a slight downward component to the spiraldirection. As discussed above, the vertical orientation of generatorpairs will most likely be the most useful. If desired multiple pairs, asin FIG. 1, may be employed to simulate conditions caused by multiplelifting surfaces or multipleaircraft.

Referring now to FIG. 2 of the drawingswherein an embodiment of thetrailing vortex generator is shown. Vortex generator means 6 such as apair of vortex generators 8 and 12 are mounted vertically at the end.

of a three-segment airfoil-shaped hollow arm 4 on a support strut 9. Asdiscussed above multiple generator pairs may be used and the generatorpairs may be mounted at other angles than the vertical. For the purposesof discussing this embodiment, a vertical orientation providingspiraling vortices directly outward will be assumed. FIG. 3 is across-sectional view of arm 4 showing the vortex generator air supplytube 40 and the boundary layer suction flow passageway 38 that actsthrough a plurality of small holes 41 locatednear the trailing edge ofthe airfoil surfaces to provide conventional boundary layer suctionaction. Arm 4 is attached to a rotatable vertical shaft 2 havingcounterweights 22 and 24 attached from arm 4. Arm 4 is tapered insegments for greater rigidity and strength and yet have minimumaerodynamic interference with the vortex trails. Shaft 2 may besupported by conventional framework (not shown).

Shaft 2 is rotatably driven by any conventional means. Tube 20 isconnected to tube 40 in the arm 4. A source of compressed gas orpressurized liquid (not shown) is connected to. the free end of tube 20.Tube 18 is connected to the hollow section 38 of arm 4; the free end oftube 18 is connected to a source of suction sufficient to provide theboundary layer suction.

A circular wall 32, shown in cross-section is provided around theperimeter of the testing area. Screening material 30 that functions as avortex trap is mounted on the inner perimeter of wall 32. Trap material30 viewed in cross-section, is a rectangular cage having an aperture orgap in the vicinity of the plane of rotating arm 4. Additional screeningmaterial 26 and 28 is mounted above and below rotating arm 4,respectively, to reduce the turbulence and distortion in the flowinduced outward toward the vortex generators. The screening 26, 28 and30 may be supported by any suitable means including wood framing, forexamples Screens 26 and 28 are parallel to rotating arm 4 for a lengthextending to the vortex generators 8 and 12 at its tip; at that pointscreen 26 rises vertically,.perpendicu lar to arm 4 and screen 28 dropsvertically, perpendicuare seen in cross section in this figure. Uponreaching wall 32 they move along the wall surface as shown and dissipatewithin screening 30. If desired, to simulate aircraft take off andlanding. conditions, screening 30 can be eliminated and wall 32 moved incloser to simulate a ground effect.

A sensing device or probe 52 may be held by a standard 51 in the openarea between the end of screens 26, 28, and the wall 32. Standard 51 haswheels 53 and a motor 55 driving one set of the wheels by a pulley belt57 or then suitable means so that the standard may be driven outward soas to remain at a constant location with respect to one of the outwardspiraling vortices.

Suitable probes include-those that are pressure sensitive, the pressureswould be converted to electrical signal, amplified, and recorded on apaper chart, film or the like. Or, hot wire anemometers could be usedwherein fluctuations in air flow would produce measurable changes inwire resistance. Such probes and others that would be suitable are wellknown in the art. Another type of instrumentation usable with theinvention is smoke flow visualization with photography. Smoke could beintroduced in the vortex generators or immediately downstream of thewing or generators through a probe 52 mounted on one of the supportstandards 51.

FIG. 7 shows an alternative way of holding a sensing probe 52. Anextension 54 is provided from arm 4 beyond the vortex generators andprobe 52 is mounted at the end thereof. Thus the probe will rotate inunison with the arm.

FIG. 8 shows a further alternative way of holding probe 52. A second arm56 is rotated from the axis of shaft 2. The arm may be used to guide amovable standard 58 having an upright portion 50 that is attached to arm56. As in FIG. 2, probe 52 would be mounted atop upright portion 50 ofthe probe standard.

Alternately, the moving standard 58 and upright portion 50 could beeliminated. Thus probe 52 would be attached to the end of arm 56 with nofurther support. Arm 56 could be rotated in synchromism with arm 4 orcould be rotated at a different rate in order to vary the effectivesampling location.

FIG. 4 shows a vortex generator 8 or 12 of FIG. 2 in greater detail as ahollow open-ended cylinder 11 joined to support strut 9. Tube 40 fromarm 4 blows pressurized fluid along the side wall of cylinder 11 tocause a swirling of fluid that interacts with the fluid coming in thefront end of the cylinder through screen 43 during rotation to therebygenerate a vortex.

FIG. 5 shows an alternative vortex generator, a hollow open-endedcylinder 11 with screen 43 having therein an electric motor 46 driving ashaft 44 and propellor blade 42. The rotating propellor blade actssimilarly to the jet of pressurized fluid in the embodiment of FIG. 4 togenerate a vortex as the cylinder is moved through the fluid.

FIG. 6 shows a curved wing section 48 mounted at the end of arm 4. Apair of trailing vortices will be generated as the wing is moved throughthe fluid near the extremities of the trailing edge of the wing. Thewing is curved because it is desired to produce a vortex pair closetogether; yet the strength of a vortex pair depends on the wing sectionlength. Thus folding over the wing as shown in FIG. 6 permits thegeneration of a strong closely located vortex pair.

In FIG. 9, an alternate vortex trap is shown. The outward spiralingvortices 34 and 36 (see in cross-section) generated by vortex generators8 and 12, respectively, enter the opening in screening 30, turn andtravel along the wall 32. Apertures or slits 35 are provided in wall 32for the vortices to propagate outward so as to leave the fluid enclosedby the circular enclosing wall 32. The structural details of thisalternative vortex trap form no part of the invention; conventionaltechniques in construction may be used.

Further Considerations of the Invention The induced velocity of a pairof concentrated retilinear vortices is given by where V is velocity, I,the lifting surface circulation, and a, the distance between thevortices. This equation initially gives a good approximation for thevelocity of the vortex pair generated by this facility. Having adequatespacing between vortex pairs is necessary for minimum interference. Forthis apparatus, the spacing between vortex pairs, d, is determined bywhere D is twice the radius of the arm 4, and U is its tip velocity.

The invention allows trailing vortices of extraordinary length to beobtained. An equivalent wind tunnel for vortex testing would be muchmore expensive. This invention with a 7-foot arm would produce in 1.6turns the same length vortex that could be generated in a 40- by--footwind tunnel (the largest in the world). To study the aircraft trailingvortex problem a 27 ft. diameter circular wall (32) around a 7 ft radiusarm (4) for example, operation with a tip speed of 300 ft/sec couldgenerate a spiral vortex that 2% revolutions behind the vortexgenerators was a small model of the trailing vortices 20 miles behindthe Boeing 747 airplane. The most important dimensionless parameter(velocity distance behind the airplane/airplane circulation) can beaccurately scaled.

Several questions bear discussion in connection with the vortex motion.Arc segments in a newly created vortex pair expand in length as theymove radially outward. For the extreme case of zero draft of thetrailing vortex generators, so that there is no net axial flow in thevortices, vortex lines are being stretched as the vortices move nearlyradially outward, but the circulation about each vortex remainsunaffected until physical contact between the vortices occurs. Thestretching process will tend to reduce the core diameter, but this willbe a small effect compared to the outward diffusion of vorticity. Forthe normal case, where there is an axial velocity defect, fluid movesalong the vortex back toward the generators and partially nullifies thestretching process.

Another question concerns the interaction of the vortex pairs. Examininga radial cross section of the flow out board of the vortex generator,there is a finite row of vortex pairs. The induced velocity of symmetricvortex pairs in infinite rows of rectilinear vortices is the same as foran isolated pair. An indefinite row of symmetrically arranged vortexpairs is unstable, but the vortices will maintain their relativepositions unless disturbed and then the disturbance propagates ratherslowly if the radial spacing is large compared to the lateral spacing.Also, at the beginning of the vortex pair row, the spacing between thevortices of the first ring will decrease because of end effectsuntil.they become the second ring. Thereafter their spacing remainsessentially constant. However, for a spacing ratio d/a 4, the velocitywith which they approach each other is less than 3 percent of thevelocity of the isolated vortex pair. Therefore, the decrease in thespacing of the vortex pair between the first and second rings is verysmall. Also, the vortex core size is always growing, and eventually thevorticity of the two vortices starts cancelling. This also affects thepropagation velocity. I

The limit of how long the coherent flow structure of the trailing vortexpair can be studied will be determined by the vortex breakup. Thiseffect is directly related to the'free-stream turbulence level and anyunsteadiness in the vortex generation. Starting this test in very stillroom air, the turbulence level will initially be extremely low. It willprobably rise if the turbulent wake from the rotating arm is allowed todevelop. This wake is essentially equivalent to the resultingdisturbance from the strut holding a wing in a wind tunnel. While thearm is much longer than a strut, only the outer radiussection'effectively disturbs the flow and the effect can be reduced byshielding. It is expected that boundary-layer suction can reduce thiseffect to a minimum.

From wind-tunneldesign it is well known that the most effective screento use to reduce flow disturbances. is that with a drag coefficient of2. A screen with a greater value will actually invert flow disturbances.Also, the structure should be upstream of the screens and contact withthe screens kept to a minimum of surface area. Two counterweights 22 and24 are shown outside of the screened-in region. They should also bedesigned for minimum drag.

Smoke flow visualization will be a convenient tool for visualizing thevortices to see how they diffuse and eventually break up. Also,three-dimensional flow aspects can be visualized. Here, as in the realairplane case, the flow velocities with respect to a ground observerreturn to zero, unlike those in a wind tunnel, which return to thefree-stream value. Hot wire equipment is the. most suitable method formeasuring the turbulence and low velocities in the facility.

Instrumentation can be mounted in one or more of the severalmethodsdiscussed hereinbefore. The strut extension has severaldisadvantages: additional interference, and measurements can only bemade after each whole turn, withthe need for slip rings to obtain thedata. For mean velocities, a stationary stand probably will besatisfactory since velocity averages can be obtained over many cycles ofthe vortices sweeping across the instrumentation. Obtaining theturbulent velocity information may possibly require a moving platformbecause of the low frequencies involved.

It will be apparent to those of ordinary skill in the art that theinvention as described herein is susceptable to many modificationswithout departing from the spirit and scope of the invention. Forexample, the invention is not limited to testing in the air; theinvention could be practiced in other fluids. The invention is thereforeto be limited only by the scope of the appended claims.

I claim:

1. Apparatus for generating trailing vortex pairs in a fluid mediumcomprising:

support means defining an axis in said fluid medium,

vortex pair generating means rotatably mounted about said axis forgenerating trailing vortices moving in a generally spiral pathsubstantially radially outward from said axis in said medium, and meansfor measuring fluid disturbances resulting from said trailing vorticesin said fluid medium.

2. Apparatus for'generating trailing vortex pairs in a quiescent fluidmedium, comprising:

support means defining an axis in said fluid medium,

vortex air generating means rotatably mounted about said axis forgenerating trailing vortices moving in a generally spiral path away fromsaid axis in said medium,

means independent of the vortex pair generating means for rotating saidgenerating means about said axis, and means for measuring flUlddisturbances resulting said fluid disturbances in said medium.

4. Apparatus according to claim 3 further comprising means for trappingtrailing vortices that have spiraled a predetermined distance away fromsaid axis.

5. Apparatus according to claim 4 further comprising a housingsurrounding said axis'and adapted to contain a body of said fluidmedium.

6. Apparatus according to claim 5 wherein said housing is of agenerallycircular cylindrical form, said axis centered therein and said trappingmeans is adjacent and within said housing.

7. Apparatus according to claim 3 wherein said support means and vortexgeneratingmeans comprises:

a support structure along said axis,

an arm. perpendicular to said axis and attached to said supportstructure,

a plurality of vortex generators attached to said arm and spaced fromsaid support, and

means for rotating said arm about said axis.

8. Apparatus according to claim 3 wherein said measuring meanscomprises:

sensing probe means, and

means for holding said probe means.

9. Apparatus according to claim 8 wherein said means for holdingcomprises an extension arm attached to said horizontal arm.

10. Apparatus according to claim 9 wherein said extension arm extendsradially outward from said axis.

1 1. Apparatus according to claim 9 wherein said means for holdingcomprises:

a second perpendicular arm attached to said support and rotating at apredetermined angular velocity about an axis within said fluid medium togenerate trailing vortices moving in a generally spiral path away fromsaid axis, and

measuring said fluid disturbances in said medium.

15. The method of claim 14 further comprising trapping vortices thathave spiraled a predetermined distance away from said circular path.

1. Apparatus for generating trailing vortex pairs in a fluid mediumcomprising: support means defining an axis in said fluid medium, vortexpair generating means rotatably mounted about said axis for generatingtrailing vortices moving in a generally spiral path substantiallyradially outward from said axis in said medium, and means for measuringfluid disturbances resulting from said trailing vortices in said fluidmedium.
 2. Apparatus for generating trailing vortex pairs in a quiescentfluid medium, comprising: support means defining an axis in said fluidmedium, vortex air generating means rotatably mounted about said axisfor generating trailing vortices moving in a generally spiral path awayfrom said axis in said medium, means independent of the vortex pairgenerating Means for rotating said generating means about said axis, andmeans for measuring fluid disturbances resulting from said trailingvortices in said medium.
 3. Apparatus for generating trailing vortexpairs and for measuring the resulting fluid disturbances in a fluidmedium comprising: support means defining an axis in said fluid medium,vortex pair generating means rotatably mounted about said axis forgenerating trailing vortices moving in a generally spiral path away fromsaid axis in said medium, and measuring means for measuring said fluiddisturbances in said medium.
 4. Apparatus according to claim 3 furthercomprising means for trapping trailing vortices that have spiraled apredetermined distance away from said axis.
 5. Apparatus according toclaim 4 further comprising a housing surrounding said axis and adaptedto contain a body of said fluid medium.
 6. Apparatus according to claim5 wherein said housing is of a generally circular cylindrical form, saidaxis centered therein and said trapping means is adjacent and withinsaid housing.
 7. Apparatus according to claim 3 wherein said supportmeans and vortex generating means comprises: a support structure alongsaid axis, an arm perpendicular to said axis and attached to saidsupport structure, a plurality of vortex generators attached to said armand spaced from said support, and means for rotating said arm about saidaxis.
 8. Apparatus according to claim 3 wherein said measuring meanscomprises: sensing probe means, and means for holding said probe means.9. Apparatus according to claim 8 wherein said means for holdingcomprises an extension arm attached to said horizontal arm. 10.Apparatus according to claim 9 wherein said extension arm extendsradially outward from said axis.
 11. Apparatus according to claim 9wherein said means for holding comprises: a second perpendicular armattached to said support and rotating at a predetermined angularvelocity with respect to said first recited perpendicular arm, saidsecond arm having a greater length than said first recited perpendiculararm.
 12. Apparatus according to claim 11 wherein said second arm rotatesat the same angular velocity as said first recited arm and said arms arespaced by a predetermined angle.
 13. Apparatus according to claim 8wherein said means for holding comprises a support structure movableradailly from said axis.
 14. A method of generating and measuring fluiddisturbances in a fluid medium comprising: rotating a plurality ofvortex generators in a path about an axis within said fluid medium togenerate trailing vortices moving in a generally spiral path away fromsaid axis, and measuring said fluid disturbances in said medium.
 15. Themethod of claim 14 further comprising trapping vortices that havespiraled a predetermined distance away from said circular path.